Differential amplifiers typically have a built-in or internal DC offset due to device mismatch and parameter variations caused by manufacturing variations, as will be understood by those in the art. This offset causes asymmetry or mismatching of the amplifier components. Of particular note, the DC offset produces mismatch in the common mode voltages of the differential outputs of the amplifier. The input-referred offset voltage of an amplifier is the differential voltage required to be applied at the input of the amplifier to produce a null output. Many applications require the cancellation/minimization of the offset voltage. For example, a limiting amplifier used in broadband optical communications often requires the offset voltage to be around 0.1 mV or less. When the offset voltage is higher, the decision circuit will slice the data at a non-optimal level which leads to a sensitivity reduction and thus a poor bit-error-rate performance. A typical single stage BJT amplifier has a 3σ random offset of a few millivolts. A RF MOS amplifier typically has an offset voltage of a few 10 millivolts. The offset is much larger for multiple stage amplifiers. Therefore, offset cancellation schemes are employed to reduce the inherent offset to the desired level.
FIG. 1 is a circuit diagram of a prior art analog amplifier circuit 10 having offset cancellation. The circuit 10 includes a main operational amplifier 12 having positive and negative inputs and positive and negative outputs. The differential input voltage signal (VIP and VIN) at the inputs is AC coupled through a pair of capacitors to the main amplifier 12. A differential output signal (VON and VOP) is provided at the output nodes. The input capacitors are relatively large and are designed to remove the common mode DC component from the input data signal. The circuit 10 also includes an error operational amplifier 14 and two low-pass RC filters coupled between the outputs of the main amplifier 12 and the inputs of the error amplifier 14. The error amplifier 14 supplies a differential DC input voltage to the input of main amplifier 12 to compensate for the main amplifier's offset voltage. The error amplifier 14 senses the DC component from the main amplifier's output signal using the two low-pass RC filters and adjusts its output voltage until the main amplifier's differential output voltage is compensated. The output impedance of the error amplifier 14 together with the 50Ω output resistors serve as the input termination of the main amplifier 12.
There are two reasons why the circuit 10 does is not completely eliminate the offset voltage: (i) the finite gain of the error amplifier 14 and (ii) the offset voltage VOS1 of the error amplifier. A simple analysis shows that the main amplifier 14 offset voltage is reduced to:
                              V          OS          ′                =                                                            V                OS                            +                                                A                  1                                ⁢                                  V                                      OS                    ⁢                                                                                  ⁢                    1                                                                                                      A                ×                                  A                  1                                            +              1                                ≈                                                    V                OS                                            A                ×                                  A                  1                                                      +                                          V                                  OS                  ⁢                                                                          ⁢                  1                                            A                                                          (        1        )            Since the error amplifier 14 does not have to be fast, large transistors with good matching properties can be used to make VOS1 very small. Depending on the amount of the offset that must be removed, the gain of the error amplifier 14 A1 can be a buffer (A1=1) or an amplifier (A1>1). Typically, a buffer is sufficient for a BJT amplifier while MOS amplifiers require additional loop-gain to meet the offset cancellation.
The offset-compensation circuit of FIG. 1 does not only suppress the offset voltage, but also the low-frequency components of the input signal. This undesired effect leads to a low frequency cutoff in the main amplifier 12's frequency response. The 3-dB low-frequency cutoff due to the offset compensation is
                              f          LF                =                              1                          2              ⁢                                                          ⁢              π                                ⁢                                                    A                ×                                                      A                    1                                    /                  2                                            +              1                                      R              ×              C                                                          (        2        )            From this equation, it can be seen that in order to get a low cutoff frequency, we need to make the loop bandwidth 1/(2π×RC) much smaller. For example, if A×A1/2=100, we need a loop bandwidth of 10 kHz to achieve a cutoff frequency of 1 MHz in the main amplifier 12. As a result, the resistance and capacitance used in the RC network are usually very large, occupying excessive and often unacceptable amounts of chip area.